Sequenced pulse reverse waveform surface finishing of additively manufactured parts

ABSTRACT

A method of and system for surface finishing an additive manufactured part. A part having a surface roughness with macroasperities is placed in a chamber with an electrolyte and an electrode. A pulse/pulse reverse power supply is connected to the part rendering it anodic and connected to the electrode rendering it cathodic. The power supply is operated to decrease the surface roughness of the part by applying a first series of waveforms including at least two waveforms where a diffusion layer is maintained at a thickness to produce a macroprofile regime relative to the macroasperities, the first series of waveforms having anodic voltages applied for anodic time periods before cathodic voltages applied for cathodic time periods to effect part surface smoothing to a first surface roughness with minimal material removal and applying a final waveform where the diffusion layer represents a microprofile regime, the final waveform having a final anodic voltage applied for a final anodic time period before a final cathodic voltage applied for a final cathodic time period to effect part surface smoothing to a final surface roughness with minimal material removal.

RELATED APPLICATIONS

This application claims benefit of and priority to U.S. ProvisionalApplication Ser. No. 62/829,191 filed Apr. 4, 2019, under 35 U.S.C. §§119, 120, 363, 365, and 37 C.F.R. § 1.55 and § 1.78, which isincorporated herein by this reference.

FIELD OF THE INVENTION

This subject invention relates to high rate electrochemical surfacefinishing of metal parts.

BACKGROUND OF THE INVENTION

Metal additive manufacturing (AM) or three-dimensional (3-D)manufacturing processes have the potential to revolutionize the art ofmanufacturing complex components enabling a build to print scenario.Herein, AM and 3-D are used interchangeably to describe thesemanufacturing processes. One type of additive manufacturing is thepowder bed fusion (PBF) process which includes the following commonlyused printing techniques: Direct Metal Laser Sintering (DMLS), ElectronBeam Melting (EBM), Selective Heat Sintering (SHS), Selective LaserMelting (SLM) and Selective Laser Sintering (SLS).

PBF methods use either a laser or electron beam to melt and fusematerial powder together. EBM methods require a vacuum but can be usedwith metals and alloys in the creation of functional parts. All PBFprocesses involve the spreading of the powder material over previouslayers. There are different mechanisms to enable this including a rolleror a blade. A hopper or a reservoir below or adjacent the bed providesfresh material supply. DMLS is the similar to SLS, but with the use ofmetals and not plastics. The process sinters the powder layer by layer.SHS differs from other processes by using a heated thermal print head tofuse powder material together. As before, layers are added with a rollerin between fusion of layers. A platform lowers the part accordingly.

The PBF process steps generally include a layer, typically 0.05 to 0.25mm thick of material spread over the build platform. A laser or electronbeam then fuses the first layer or first cross section of the model. Anew layer of powder is spread across the previous layer using a rake.Further layers or cross sections are fused and added. Then the processrepeats until the entire part is created. Loose, unfused or partiallysintered powder remains in position but is removed during postprocessing.

Other metal additive manufacturing processes use different feedstockforms for the supply material. These processes generally include WireDirected Energy Deposition such as Laser Metal Deposition-wire (LDM-w)and Powder Directed Energy Deposition such as Laser Engineering NetShape (LENS), Laser Metal Deposition-powder (LMD-p).

Metal additive manufacturing processes can cause microstructuralanisotropy/inhomogeneity, porosity (open near the surface and closedinternally), surfaces with partially sintered materials and largesurface roughness. Therefore, at the completion of the metal additivemanufacturing process, the resulting parts are commonly hotisostatically pressed (HIPped) to eliminate sub surface (enclosed)porosity and machined or otherwise finished to eliminate any partiallyconsolidated surface material or near surface porosity from the surface.This surface porosity can be detrimental to the material performance ofa wide range of applications and must be removed prior to materialusage.

BRIEF SUMMARY OF THE INVENTION

Provided is an improved surface finish method and system ofthree-dimensional (3-D) additively manufactured (AM) parts andcomponents made from powder bed fusion process including direct metallaser sintering (DMLS), electron beam melting (EBM), selective heatsintering (SHS), selective laser melting (SLM) and selective lasersintering (SLS) or wire directed energy deposition such as laser metaldeposition-wire (LDM-w) and powder directed energy deposition such aslaser engineering net shape (LENS), and laser metal deposition-powder(LMD-p). In addition to large surface roughness, parts made from 3-D/AMprocesses generally contain an unacceptable high surface porosity evenafter HIPed. A large surface roughness is electrochemically finished toan acceptable surface roughness whereby the surface may include bothexterior and interior surfaces. Preferably, a first set of waveformparameters are tuned to achieve surface smoothing from a predeterminedfirst starting surface roughness to a first final surface roughness withminimal material removal. Then, a second set of waveform parameters aretuned to achieve surface smoothing from said first final surfaceroughness to a second final surface roughness with minimal materialremoval. The waveform may be tuned or sequenced a sufficient number oftimes to achieve the desired final surface finish with minimal materialremoval.

The preferred method includes high rate electrochemical surfacefinishing of internal and external surfaces of metal parts andcomponents with highly rough and/or porous surfaces. The process isapplied to additively manufactured metal parts with highly rough initialsurfaces to a final roughness encompassing a wide range of roughnessvalues.

The process can be applied to high rate electrochemical surfacefinishing of additively manufactured metal parts made from alloysincluding INCONEL 718, HASTELLOY® X, Ti6Al4V, and other materials.

The surface roughness (Ra) is reduced while at the same time the amountof material removed is minimized. Large surface disparities are removedand other processes, like vibratory finishing, can be used to removefiner surface disparities.

Consequently, the surface (internal and external) porosity and roughnessof additive manufactured metal parts and components are improved in anefficient and cost-effective manner while minimizing the amount ofmaterial removed.

In some embodiments, the instant invention addresses the problem ofsurface finishing of additively manufactured metal parts and componentswith a large initial roughness by tuning a first set of waveformparameters to effect surface smoothing to a predetermined first finalsurface roughness with minimal material removal. Next, the waveform issequenced to a second set of waveform parameters to achieve surfacesmoothing from said first final surface roughness to a second finalsurface roughness with minimal material removal. And still next, thewaveform is sequenced to a third set of waveform parameters to achievesurface smoothing from said second final surface roughness to a thirdfinal surface roughness with minimal material removal. The waveform issequenced a sufficient number of times until the macro- and/ormicroasperities are removed to achieve the desired final surface finishwith minimal material removal.

See also U.S. Pat. Nos. 6,402,931; 6,558,231; 7,022,216; 9,006,147; and9,987,699, all of which are incorporated herein by this reference.

In one embodiment of the invention, the initial surface roughness of anadditively manufactured powder bed fusion metal part or component is atleast 50 microns (2000 microinches). In another embodiment of theinvention, the initial surface roughness of an additively manufacturedpowder bed fusion metal part or component is at least 25 microns (1000microinches). Instill another embodiment of the invention, the initialsurface roughness of an additively manufactured powder bed fusion metalpart or component is at least 5 microns (200 microinches). In stillanother embodiment of the invention, the reduction in surface roughnessof an additively manufactured powder bed fusion metal part or componentfrom initial roughness to final roughness after sequenced pulse reversewaveform electrofinishing is at least 10×. In still another embodimentof the invention, the reduction in surface roughness of an additivelymanufactured powder bed fusion metal part or component from initialroughness to final roughness after sequenced pulse reverse waveformelectrofinishing is at least 5×. In still another embodiment of theinvention, the reduction in surface roughness of an additivelymanufactured powder bed fusion metal part or component from initialroughness to final roughness after sequenced pulse reverse waveformelectrofinishing is at least 3×. In still another embodiment of theinvention, the surface of an additively manufactured powder bed fusionmetal part or component is an internal surface and the frequency isadjusted such that a maximum of internal surface is smoothed. In stillanother embodiment of the invention, a finishing process can be used toreduce the overall roughness of the additively manufactured powder bedfusion metal part can be used to accelerate finishing or front-end otherfinishing processes like, vibratory finishing.

Featured is a method of and system for surface finishing an additivemanufactured part one preferred method includes placing the part havinga surface roughness with macroasperities in the chamber with anelectrolyte and an electrode, connecting a pulse/pulse reverse powersupply to the part rendering it anodic and to the electrode rendering itcathodic, and operating the power supply to decrease the surfaceroughness of the part. A first series of waveforms are applied includingat least two waveforms where a diffusion layer is maintained at athickness to produce a macroprofile regime relative to themacroasperities, the first series of waveforms having anodic voltagesapplied for anodic time periods before cathodic voltages applied forcathodic time periods to effect part surface smoothing to a firstsurface roughness with minimal material removal. A final waveform isapplied where the diffusion layer represents a microprofile regime, thefinal waveform having a final anodic voltage applied for a final anodictime period before a final cathodic voltage applied for a final cathodictime period to effect part surface smoothing to a final surfaceroughness with minimal material removal. The first series of waveformtimes may be between 1 millisecond and 100 milliseconds, and the finalwaveform time may be between 1 millisecond and 100 milliseconds. Theanodic voltages are preferably between 3 volts and 40 volts. The firstseries of waveform anodic time periods are preferably between 0.1millisecond and 50 millisecond and the final anodic time period ispreferably between 50 milliseconds and 100 milliseconds. The cathodicvoltages are preferably between 4 volts and 30 volts. The cathodic timeperiods may, in one embodiment, replace at least some or all of theoff-times between the anodic times. The first surface roughness ispreferably between 5 microns and 50 microns, the final surface roughnessis preferably between 0.5 microns and 15 microns and the total amount ofmaterial removed is preferably between 50 microns and 250 microns.

The subject invention, however, in other embodiments, need not achieveall these objectives and the claims hereof should not be limited tostructures or methods capable of achieving these objectives.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a generalized pulse revere waveform;

FIG. 2 is a representation delineating the pulsating versus stationarydiffusion layer for an anodic process;

FIGS. 3A and 3B are a representation of the impact of long and shortpulse on-times under conditions of macroprofile (FIG. 3A) andmicroprofile (FIG. 3B);

FIG. 4 are guidelines for designing pulse parameters as a function ofmacroprofile/microprofile and need for more uniformity or morenon-uniformity;

FIG. 5 is a plot of average surface finish versus average thickness ofmaterial removed for Waveform 1 and Waveform 2;

FIG. 6 presents the calculated etch depth as a function of time andwaveform for sample B320_2_60 using 10% H₂SO₄ solution;

FIG. 7 illustrates the change in surface roughness (R_(a)) as a functionof time and waveform for sample B320_2_60 in a 10% H₂SO₄ solution;

FIG. 8 are 3D surface maps of IN 718 sample B320_2_60 before processing(left) and after processing with waveform sequence for 40 minutes(total) in 10% H₂SO₄ solution;

FIG. 9 are photographs showing polishing evolution from sample B320_2_60using waveform sequence in 10% H₂SO₄;

FIG. 10 is the etch profile (Blue is deep, Red is shallow) found whenfinishing AM Ti6Al4V in 30 w/w % H_(Z)SO₄ and 150 g/L Na₂SO₄ electrolytewith the conditions shown in TABLE V;

FIG. 11 is the etch profile (Blue is deep, Red is shallow) found whenfinishing AM Inconel 718 in 0.23 M HCl and 0.23 M Citric Acidelectrolyte with the conditions shown in TABLE VII; and

FIG. 12 is a view of an example of an electrochemical surface finishingsystem in an example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

FIG. 1 shows a generic pulse reverse current waveform for a net anodicor electrofinishing process. The generic description illustrates aanodic (forward) pulse followed by an anodic off-time, followed by ancathodic (reverse) pulse and followed by an cathodic off-time. Theanodic peak current density (i_(anodic)), anodic on-time (t_(anodic)),and anodic off-time (t_(off,anodic)) cathodic peak current density(i_(cathodic)), cathodic on-time (t_(cathodic)), cathodic off-time(t_(off,cathodic)), are individual variables for process control. Whilethe terminology pulse/pulse reverse current is often employed, thewaveforms are often controlled in terms of the voltage and could also betermed pulse/pulse reverse voltage. The sum of the anodic on-time,anodic off-time, cathodic on-time, and cathodic off-time(s) is theperiod (T) of the pulse and the inverse of the period is the frequency(f). Specifically,T=(t _(anodic))+(t _(off,anodic))+(t _(cathodic))+(t_(off,cathodic))  (1)f=(1/T)  (2)

The anodic duty cycle (γ_(anodic)) is the ratio of the anodic on-time tothe pulse period, and the ratio of the cathodic on-time to the pulseperiod is the cathodic duty cycle (γ_(cathodic)). The frequency and dutycycles are additional variables for process control. The average currentdensity (i_(average)) or electrofinishing rate is given by:i _(average)=(i _(anodic))(γ_(anodic))−(i_(cathodic))(γ_(cathodic))  (3)It should be noted that even though pulse current and pulse reversecurrent (PC/PRC) waveforms contain off-times and anodic periods, the netelectrofinishing rate is often the same or higher than in direct current(DC) processing. As discussed below, this is attributed to the fact thatthe “instantaneous” peak currents attained during the pulse on-time canbe much higher than that attained during DC processing.

Mass transport in pulse current processing is a combination of steadystate and non-steady state diffusion processes. The mass transferlimited current density (i_(l)) is related to the reactant concentrationgradient (C_(b)−C_(s)) and to the diffusion layer thickness (δ) by:i _(l) =−nFD(∂C/∂x)_(x=0) =−nFD[(C _(b) −C _(x))/δ]  (4)where n, F, D are the number of equivalents exchanged, Faraday'sconstant, and diffusivity of the reacting species, respectively. In DCelectrolysis, δ is a time-invariant quantity for a given electrodegeometry and hydrodynamic condition. In pulsed electrolysis, however, δvaries from 0 at the beginning of the pulse to its steady state valuewhen the Nernst diffusion layer, δ_(N), is fully established. Thecorresponding mass transport limiting current density is equal to aninfinite value at t=0 and decreases to a steady state value of the DClimiting current density. The advantage of pulse electrolysis is thatthe current is interrupted before δ reaches steady state, allowing thedissolved ions to diffuse away from the surface and partially orcompletely reduce the surface concentration before the next currentinterruption. Therefore, the concentration of dissolved ions in thevicinity of the electrode changes with the pulse frequency. During pulseelectrolysis, a “duplex diffusion layer” includes a pulsating layer,δ_(p), and a stationary layer, δ_(s) for a deposition process. FIG. 2shows the equivalent diffusion layers for an anodic process, e.g. metalremoval. By assuming a linear concentration gradient across thepulsating diffusion layer and conducting a mass balance, the pulsatingdiffusion layer thickness (δ_(p)) is:δ_(p)˜(2Dt _(on))^(1/2)  (5)where t_(on) is the pulse on-time. When the pulse on time is equal tothe transition time, the concentration of reacting species at theinterface drops to zero at the end of the pulse. An expression for thetransition time, τ, is:τ˜((nF)² C _(b) ² D)/2i _(c) ²  (6)

For on-times less than the transition time, the concentration ofdissolving species at the interface is low. For on-times equal to orgreater than the transition time, the concentration of reacting speciesis high and mass transport becomes an important consideration.

For a DC process, a hydrodynamic Nernst diffusion layer is established.The thickness of the pulsating diffusion layer is related to the pulsecurrent on-time and we refer to it as the “electrodynamic” diffusionlayer. The key points used in the development of a pulsed process forelectrodeposition are:

(1) the electrodynamic diffusion layer thickness is proportional topulse on time, and

(2) transition time is inversely proportional to the pulse current.

Current distribution is an important parameter in electrochemicalsurface finishing processes. Primary current is governed solely by thegeometric effects of the electrochemical cell. Secondary currentdistribution is governed by kinetic effects and activationoverpotentials are considered. Tertiary current distribution is governedby mass transport effects and both activation and concentrationoverpotentials are considered. The addition of secondary or tertiarycurrent distribution effects tend to make the current distribution moreuniform, as compared to primary current distribution alone. In the caseof surface finishing to reduce surface roughness, the currentdistribution should be focused on the peaks or asperities in order topreferentially remove the surface roughness.

Another important consideration is the relationship between thethickness of the hydrodynamic diffusion layer and the surfaceasperities. For a given electrolyte, the thickness of the hydrodynamicdiffusion layer is determined by the degree of solution agitation in theelectrochemical cell. Specifically, with high solution agitation thethickness of the hydrodynamic diffusion layer is smaller than the caseof low solution agitation. The degree of solution agitation isdetermined by factors understood by those skilled in the art and includesolution flow rate, stir bars and paddles, and the like. For a givenelectrochemical cell, the solution agitation is generally constant andconsequently the thickness of the hydrodynamic diffusion layer isconstant during surface finishing of the workpiece.

In a macroprofile (FIG. 3A), the roughness of the surface is largecompared with the thickness of the hydrodynamic diffusion layer, δ_(H),and when a pulsed electric field is applied, the diffusion layer iscompressed to form a pulsating or electrodynamic diffusion layer, δ_(p).The pulsating diffusion layer, δ_(p), tends to follow the surfacecontour, and becomes more compressed and thinner as the pulse on-timebecomes shorter. In a microprofile (FIG. 3B), the roughness of thesurface is small compared with the thickness of the hydrodynamicdiffusion layer, δ_(H). In this case, for a long pulse on-time, thepulsating diffusion layer is compressed, but still is much larger thanthe characteristic length, and the microprofile is maintained. For veryshort pulse on-times, the pulsating diffusion layer is compressed to thepoint at which it follows the surface contour, and the systemeffectively mimics a macroprofile.

A final consideration is that if the applied waveform is designed suchthat the pulse on time is much longer than the transition time, tertiarycurrent distribution will play an important role. With the addition oftertiary control, the concepts of macroprofile and microprofile andtheir influence on current distribution become important. Under DCconditions and mass transport control, a macroprofile results in themost uniform current distribution and a nearly conformal surfacefinishing profile. In other words, the surface roughness remains thesame and is not reduced. The application of pulse currents generates asmaller macroprofile. Based on experimental observations, for amacroprofile boundary layer condition, relatively long pulse on-timescan yield a slightly non-uniform current distribution compared to DCconditions, and relatively short pulse on-times can yield asignificantly more non-uniform current distribution than DC conditions.Assuming the same average current, for shorter pulse on-times therelative influence on current distribution shifts from tertiary currentdistribution control to secondary as well as primary currentdistribution control. Consequently, as concentration polarizationeffects are removed, the current distribution becomes less uniform.

Under DC conditions and mass transport control, a microprofile resultsin the most non-uniform current distribution and a preferential removalof the surface asperities. The application of pulse currents with asmall enough on-time can convert a microprofile to a macroprofile,establishing a small δ_(p). For a microprofile diffusion layercondition, assuming tertiary current distribution control is maintained,short pulse on-times sufficient to convert the microprofile to amacroprofile results in a significantly more uniform currentdistribution. Conversely, long pulse on-times sufficient to maintain themicroprofile results in a slightly more uniform current distributioncompared to DC, assuming tertiary control is maintained by selectingon-times and peak currents that ensure t_(on)»τ.

A paradigm was developed for waveform parameter selection in terms ofcurrent distribution for macroprofile and microprofile condition. Thereare four pulse waveform types, independent of anodic or cathodicorientation that we generally use to tune waveforms for specificapplications, and these are summarized in FIG. 4.

Due to the large roughness scales encountered in additively manufacturedparts, generally the boundary layer conforms to the surface andrepresents a macroprofile. As the roughness of the workpiece decreasesduring processing, the thickness of the boundary layer remains the same.If the anodic voltage is applied in short pulses, the Nernst diffusionlayer will be thinner than it is under DC electrolysis conditions,because the full thickness of the layer does not have time to developbefore the pulse terminates. Consequently, a macroprofile condition canbe converted into a smaller macroprofile condition, thereby removing thedegree of concentration polarization or secondary current distribution.As a general rule the shorter the pulse used, the more the currentdistribution is determined by the electrode geometry (primary currentdistribution controlled by ohmic effects) and variation in overpotentialdue to electrode profile (secondary current distribution controlled bykinetic effects). In fact, depending on the size of the asperity, therole of electrochemical cell geometry may by minimal. In either case,when polarization is removed, i.e., from concentration to secondaryand/or primary, the current distribution becomes more non-uniform.Accordingly, metal will be removed preferentially at the peaks of themacroasperities whereby the surface is smoothed on a macroscopic level.When the macroasperities become microasperities, the distribution of theelectrolytic activity is influenced by the rate of mass transfer throughthe diffusion layer, which causes the effect of the electric current tobe more uniform (tertiary current distribution). Consequently, smoothingof a surface with microasperities requires a long anodic pulse on-time,or even a DC field is sufficient in cases where there is no need for areversing pulse as taught in U.S. Pat. No. 3,654,116 incorporated hereinby this reference.

In summary, the Nernst diffusion layer thickness is constant duringsurface finishing. Additionally, the surface asperities are becomingsmaller during surface finishing. In the initial stages of surfacefinishing, the relationship of the Nernst diffusion layer thickness tothe surface asperities represent a macroprofile. Depending on the finalsurface roughness required, at the conclusion of the surface finishingprocess, the relationship of the Nernst diffusion layer thickness to thesurface asperities is either a smaller macroprofile or a microprofile.Due to the changes in the relationship of the Nernst diffusion layerthickness to the surface asperities during processing, the need toadjust the waveform parameters will arise in order to achieve thedesired final surface finish with minimal material removed. Combiningmultiple waveforms into multiple successive sequences may also berequired, as will be shown in the examples to follow. This can beaccomplished with modern programmable rectifiers—rather than having tomodify electrolytes or electrode geometries to achieve the same effect.For cases where the final relationship between the Nernst diffusionlayer thickness and the surface asperities represents a macroprofile,the anodic on-times are decreased with each successive waveformsequence. For cases where the final relationship between the Nernstdiffusion layer thickness and the surface asperities represent amicroprofile, the anodic on-time is greater than the anodic on-times ofany of the preceding steps.

The anodic pulse voltages are material specific and determined bymeasurements of current as a function of voltage to determine thebreakdown voltage as indicated by the rapidly rising current. Thecathodic voltage is also material specific and is required to remove anoxide film formed during the application of the anodic voltage asdescribed in U.S. Pat. No. 6,402,931 incorporated herein by thisreference. The anodic and cathodic voltages are adjusted to account forthe distance between the workpiece and the tool.

Example 1

In this example, two waveforms were applied for electrofinishing ofcoupons of HASTELLOY® X. The parameters of the waveforms are presentedin TABLE I. The initial surface roughnesses of the coupons as measuredby R_(a) was approximately 10 μm (390 μin). After processing to adesired final surface finish of approximately 2 μm (80 μin), bothWaveform 1 and Waveform 2 removed approximately 140 μm (5,500 μin) ofmaterial as shown in FIG. 5. The time for processing with Waveform 1 was7 min and for Waveform 2 was 4 min. As noted from FIG. 5, Waveform 1exhibited a large ΔR_(a)/Δmaterial during the surface finish range of 10μm (390 μin) to 6 μm (240 μin) R_(a). As noted from FIG. 5, Waveform 2exhibited a large ΔR_(a)/Δmaterial during the surface finish range of 6μm (240 μin) to 2 μm (80 μin) R_(a).

TABLE 1 V_(anodic) t_(anodic) t_(anodic, off) V_(cathodic) t_(cathodic)t_(cathodic, off) (V) (msec) (msec) (V) (msec) (msec) Waveform 1 40 0.10 30 0.1 0 Waveform 2 40 0.7 0 20 0.5 0

In TABLE II presents the results of processing with Waveform 1 andWaveform 2 individually and with an example sequence as described in theinstant invention. By sequencing from Waveform 1 for 4 minutes followedby Waveform 2 for 1 minutes, the surface roughness was reduced from 10μm (390 μin) to approximately 2 μm (80 μin) with only 80 μm (3,150 μin)of material removed. Consequently, in this example, by sequencing thewaveform, the amount of material removed was only approximately 60% ofthe material removed using either Waveform 1 or Waveform 2 separately.

TABLE II Wave- Wave- form form Initial Final Material 1 Time 2 TimeR_(a) R_(a) Removed (min) (min) (μ/μin) (μm/μin) (μm/μin) Waveform 1 79.3/366   2.5/98.43 135/5114 Waveform 2 4 12.8/504  2.4/95 180/7086Waveform 1 + 4 1 7.1/280 1.1/43  80/3150 Waveform 2

Example 2

In this example, two waveform sequences were applied with differentallotted times for electrofinishing of coupons of Ti6Al4V. Theparameters of the waveforms used in the sequences are presented in TABLEIII. The beginning surface roughnesses of the coupons as measured byR_(a) was approximately 7 μm (275 μin). The waveform sequences consistedof Waveform 3 followed by Waveform 4 followed by Waveform 5. The datafrom Trial A are presented in TABLE IV. With the waveform sequence inTrial A, the initial surface roughness was 7.0/276 (μm/μin) R_(a) andthe final surface roughness was 1.98/78 (μm/μin) Re with materialremoved of 664/26141 (μm/μin).

TABLE III V_(anodic) t_(anodic) t_(anodic, off) V_(cathodic)t_(cathodic) t_(cathodic, off) (V) (msec) (msec) (V) (msec) (msec)Waveform 3 6 0.5 0 12 0.7 0 Waveform 4 6 0.3 0 12 0.6 0 Waveform 5 6 0.20 12 0.4 0

TABLE IV TRIAL A Total Processing Initial Final Material Material TimeR_(a) R_(a) Removed Removed (hr) (μm/μin) (μm/μin) (μm/μin) (μm/μin)Waveform 4 7.0/276 4.9/193  266/10472 3 Plus Waveform 2 4.9/193 NA NA 3Waveform 2 NA 3.7/126 233/9173 499/19645 4 Plus Waveform 6 3.7/126 NA NA4 Waveform 2 NA 1.98/78  165/6496 664/26141 5 664/26141

Based on the data obtained during Trial A, the respective time of thewaveform sequence was adjusted in order to minimize the amount ofmaterial removed as presented in TABLE V for Trial B. With the waveformsequence in Trial B, the initial surface roughness was 6.6/260 (μm/μin)R_(a) and the final surface roughness was 1.7/67 (μm/μin) R_(a) withmaterial removed of 152/5984 (μm/μin). By adjusting the time of theindividual components of the waveform sequence, we achieved anacceptable final surface finish with approximately 23% of the materialremoved in Trial A.

TABLE V TRIAL B Total Processing Initial Final Material Material TimeR_(a) R_(a) Removed Removed (hr) (μm/μin) (μm/μin) (μm/μin) (μm/μin)Plus Waveform 5 4 6.6/260 NA NA Waveform 3 3 NA 4.8/189 71/2795  71/2795Plus Waveform 4 3 NA 3.4/130 26/1023  97/3818 Plus Waveform 5 5.13.4/130 1.7/67  55/2165 152/5984 152/5984

Example 3 [IN718]

Electropolish of sample B320_2_60 was processed with the waveformsdeveloped for EMB HIP IN 718 coupon B315_2_60 using 10% (w/w) H₂SO₄electrolyte. For this coupon, Faraday varied the applied time for eachof the five waveforms to enable development of an appropriately timedwaveform sequence. As such, Faraday began polishing the SLM HIP IN 718surfaces using the conditions of waveform N, pulling the sample everytwo minutes to evaluate the surface roughness, and mass loss. The datacollected for this sample is presented in TABLE VI. FIGS. 6 and 7 showthe calculated etch depth with time and change in R_(a) with time,respectively. The 3D images collected from the Nanovea opticalprofilometer are given for this sample in FIG. 8, including initialsurface and surface after processing with the waveforms outlined inTABLE VII, and sample photographs showing the polishing evolution isgiven in FIG. 9.

TABLE VI V_(anodic) ta_(nodic) t_(anodic, off) V_(cathodic) t_(cathodic)t_(cathodic, off) (V) (msec) (msec) (V) (msec) (msec) Waveform N 10 6.01.0 12 5.0 0 Waveform O 10 1.2 1.0 12 1.0 0 Waveform P 10 0.6 1.0 12 0.50 Waveform R 10 0.6 0 10 0.5 0 Waveform T 10 0.75 0.5 12 0.6 0

TABLE VII Data generated for sample B320_2_60 processed underFARADAYIC ® Electropolishing conditions in 10% H₂SO₄ solution. OpticalContact Calculated R_(a) R_(a) R_(a) Sample ID Depth (μm) (μm) (μm) EAVEFreq Time Rate B320_2_60 (μm) x-axis y-axis y-axis (V) (Hz) (min)(μm/min) Initial Surface — 5.23 7.39 6.24 — — — — After waveform 92 — —3.31 0.0 83 6 15 N After waveform 106 — — 1.40 0.0 313 10 11 O Afterwaveform 30 — — 1.63 0.0 541 2 15 P After waveform 81 — — 1.21 0.0 476 810 R After waveform 127 0.643 0.745 0.61 0.0 541 14 9 T

The conditions developed for polishing EMB HIP IN 718 coupons in 10%(w/w) H₂SO₄ appear to also be valid for the polishing of SLM HIP IN 718coupons in 10% (w/w) H₂SO₄. After application of this waveform sequence,the surface of the SLM HIP IN 718 coupon is smoothed and has a bright,reflective and lustrous appearance. From the data presented in TABLEVII, the application of Waveform P did not improve the surface finish,at least in the order at which it was applied. The data collected fromcoupon B320_2_60 was used to generate a waveform sequence, includedapplied times for each waveform for an uninterrupted polishing step.

Example 4 [Ti6Al4V]

In this example, three waveforms of identical anodic and cathodicvoltages with varying frequencies were applied to uniformly removematerial from the internal surface of a tapered rounded rectangularTi6Al4V AM component. The parameters of the waveforms used for improvedetch uniformity of AM Ti6Al4V are presented in TABLE VIII in a 30 w/w %H₂SO₄/150 g/L Na₂SO₄ electrolyte.

TABLE VIII V_(anodic) t_(anodic) t_(anodic, off) V_(cathodic)t_(cathodic) t_(cathotic, off) (V (msec) (msec) (V) (msec) (msec) High f4 0.2 1 8 0.4 0 Mid f 4 0.8 5 8 1.6 0 Low f 4 1.2 5 8 2.0 0

The effect of these processing conditions is highlighted in FIG. 10,which show the depth of etch as a function of the process frequency andthe position of the electrodes for Ti6Al4V. The data in FIG. 10demonstrates that as the frequency increases the majority of materialremoval focusing in the region of the 10×18 mm tool.

Example 5 [IN718]

In this example, we applied three waveforms of identical anodic andcathodic voltages with varying frequencies to uniformly remove materialfrom the internal surface of a tapered rounded rectangular Inconel IN718AM component. The parameters of the waveforms used for improved etchuniformity of AM IN718 Inconel are presented in

TABLE IX in a 0.23M HCl and 0.23M Citric Acid Solution. V_(anodic)t_(anodic) t_(anodic, off) V_(cathodic) t_(cathodic) t_(cathodic, off)(V) (msec) (msec) (V) (msec) (msec) High f 3 5 0 3 10 0 Mid f 3 10 0 320 0 Low f 3 20 0 3 40 0

The effect of these processing conditions is highlighted in FIG. 11,which show the depth of etch as a function of the process frequency andthe position of the electrodes for Inconel 718. The data in FIG. 11demonstrates that as the frequency decreases the majority of materialremoval focuses in the region of the 10×13 mm tool.

One particular implementation of the disclosed electrochemical surfacefinishing system may include a working chamber 202, FIG. 12, defined bya tank 204 and a cover 208, an electrolyte holding tank 222, a conduit218, a pump 220, an electrode 304, part 302, a power source 228 and theelectrolyte solution. The working chamber 202 may be in fluidcommunication with the electrolyte holding tank 222 by way of a gravitydrain 206. A filter 224 may be associated with the drain 206 to filterthe electrolyte solution flowing from the working chamber 202 to theelectrolyte holding tank 222. The working chamber 202 may also be fluidcommunication with the electrolyte holding tank 222 by way for theconduit 218, wherein the pump 220 may pump the electrolyte solution fromthe electrolyte holding tank 222 to the working chamber 202, as shown byarrow 234.

Within the working chamber 202, a workpiece holder 210 may be mountednear the bottom of the tank 204 with adequate spacing from the walls andbottom of the tank 204 to allow for drainage of the electrolyte solutioninto the electrolyte holding tank 222. The AM part 302 may be supportedon the workpiece holder 210 and may be connected to a first electricallead 230 of pulse/pulse reverse power supply 228 rendering the partanodic. In this way, metal is dissolved from the AM part duringenergizing of the pulse/pulse reverse power supply to employ thewaveforms discussed above.

An electrode holder 214 may be positioned in the working chamber 202above the workpiece holder 210. The electrode holder 214 may be suppliedwith the electrolyte solution by way of the conduit 218. The electrode304 may be connected to a second electrical led 232 (opposite polaritythan the first electrical lead 230) of the power source 228 and may besupported by the electrode holder 214 such that the tool holder 214 maymove the electrode 304 in a vertical axis under control of an electrodefeed controller 226. Thus, electrode 304 is rendered cathodic.

In one particular aspect, the electrode 304 may include a central boreand the electrode 304 may be connected to the electrode holder 214 suchthat the central bore of the electrode 304 is directed at the workpiece302. During electrochemical surface finishing, the electrolyte solutionmay be pumped by pump 220 from the electrolyte holding tank 222 to theelectrode holder 214 and, ultimately, to the electrode 304 by way of theconduit 218. The rate of the electrolyte solution flow is hereinreferred to as E. The electrolyte solution may flow through the centralbore of the electrode 304 and may exit between the electrode 304 and thepart 302 before returning to the electrolyte holding tank 222 by way ofthe drain 206. The power source 228 may supply electric current to theworkpiece 302 and the electrode 304 by way of the first and secondelectrical leads 230, 232 in accordance with the disclosed anodicpulse-cathodic pulse waveform. The power source may be configurable toapply the waveform(s) described herein or may be controlled bycontroller subsystem 229 to apply the waveform(s). Controller subsystem229 may execute instructions stored in memory and configured to applythe disclosed waveform(s). Controller subsystem 229 may be a laptop orother computer, an application specific integrated circuit,microcontroller, field programmable gate array, or the like.

The spacing between the electrode and part 302 during processing may beconsidered a parameter subject to optimization and may depend on thecomposition of the electrolyte solution and the type of electrochemicalprocess being performed, among other factors. For example, the spacingbetween the electrode and workpiece may range from about 0.5 to 20millimeters or more particularly 0.5 to 10 millimeters for anelectrochemical shaping process, about 5 to about 12 millimeters for anelectrochemical polishing process and about 5 to about 50 millimetersfor an electrochemical deburring process.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments. Other embodiments will occur to those skilled inthe art and are within the following claims.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicantcannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

What is claimed is:
 1. A method of surface finishing, the methodcomprising: providing an additive manufactured part; placing theadditive manufactured part having a surface roughness withmacroasperities in the chamber with an electrolyte and an electrode;connecting a pulse/pulse reverse power supply to the part rendering itanodic and to the electrode rendering it cathodic; and operating thepower supply to decrease the surface roughness of the part by: applyinga first series of waveforms including at least two waveforms where adiffusion layer is maintained at a thickness to produce a macroprofileregime relative to the macroasperities, the first series of waveformshaving anodic voltages applied for anodic time periods before cathodicvoltages applied for cathodic time periods to effect part surfacesmoothing to a first surface roughness, and applying a final waveformwhere the diffusion layer represents a microprofile regime, the finalwaveform having a final anodic voltage applied for a final anodic timeperiod before a final cathodic voltage applied for a final cathodic timeperiod to effect part surface smoothing to a final surface roughness; inwhich the anodic voltages of the first series of waveforms are between 3volts and 40 volts, the cathodic voltages of the first series ofwaveforms are between 4 volts and 30 volts; and in which the firstsurface roughness is between 5 microns and 50 microns, the final surfaceroughness is between 0.5 microns and 15 microns and the total amount ofmaterial removed is between 50 microns and 250 microns.
 2. The method ofclaim 1 in which the first series of waveform times is between 1millisecond and 100 milliseconds, and the final waveform time is between1 millisecond and 100 milliseconds.
 3. The method of claim 1 in whichthe first series of waveform anodic time periods are between 0.1millisecond and 50 millisecond and the final anodic time period isbetween 50 milliseconds and 100 milliseconds.